Magnetic resonance (MR) images acquired during surgery assist a surgeon to accurately locate tissue malignancies, obtain a biopsy from desired surgical sites on the patient, and assist in the successful removal of tissue from the patient. Interventional MRI is the magnetic resonance imaging technique (often involving real-time imaging) that allows a surgeon to perform MRI-guided tissue biopsy or surgical procedures.
To provide a surgeon with open access to surgical sites during MR imaging, conventional MR reception coils often have to sacrifice their signal reception performance. For interventional MRI guided surgery, it is desirable that the MR reception coil have a large opening to receive the body part to be imaged and wide-open access to provide a surgeon access to the surgical site with a biopsy needle or other surgical devices. To provide open surgical access, conventional interventional MRI coils generally have had a simplified RF coil structure. These simplified coils have large openings and provide good access during surgery. However, these simple coils also have compromised field homogeneity and signal reception efficiency. Because of these shortcomings, conventional interventional MRI reception coils have had limited usefulness as viable surgical tools.
An example of a simplified coil configuration previously used in interventional MRI is a flat single-loop surface coil arrangement used to image breasts. The flat signal-loop coil typically is placed near the chest wall of the patient, and around all or at least a portion of a single breast. A significant disadvantage of such a single-loop coil is that it has poor field homogeneity, and provides poor signal quality, especially with respect to breast tissue not immediately adjacent the RF reception coil loop. When using a single-loop reception coil, a surgeon experiences difficulty in clearly viewing or positively identifying malignancies of the breast that are more than a short distance from the RF coil.
Improving coil performance, such as the signal-to-noise ratio and uniformity of interventional MR reception coils, would increase the depth to which tissue can be clearly imaged. If the signal reception efficiency could be improved, then signal quality would be improved and good, clear interventional MR images would be available of the surgical site and surrounding tissue. Providing a clear image of a surgical or biopsy site would allow a surgeon to make better informed decisions during MRI guided surgery.
RF quadrature detection coil can improve signal detection efficiencies effectively. A quadrature detection coil arrangement consists of two orthogonal RF reception coils. If the two coils are resonant at the same frequency, then MR signals induced in one coil (channel A) will have a 90.degree. phase shift with respect to the signals induced in the other coil (channel B).
In an MRI quadrature detection arrangement, a pair of MR signals (one from each coil-channel) are processed and combined to obtain one combined MR signal having a better signal-to-noise ratio than either of the single signals from the individual coil-channels of the quadrature arrangement. Where the two coils of the quadrature detection arrangement are identical (except for their orientation), the resultant signal should be about 40% better than that of either coil individually or of an equivalent single-loop coil arrangement.
While quadrature detection coils offer superior signal efficiency, they have in the past been difficult for use in interventional MRI devices. Providing open access to a tissue site is a significant constraint on the design of interventional quadrature detection coil. Compared to open structure single channel RF reception coils, the structure of a quadrature detection coil (having a pair of orthogonal coils) is more complex. The structural constraints of a quadrature coil arrangement present an imposing and perplexing problem which has made quadrature coils less applicable to interventional MRI, at least until the present invention.